Seismic Structure Research Articles

Will Japan Sink? ———The Explanation of the Seismo-Geothermics

Using the local earthquake catalog of Japan, based on the author’s original Seismo-Geothermics and it’s working methods, and from the study of the stereoscopic image of seismic cone structure, the independence of seismic cone tectonic activity and the inheritance of volcanic activity, this paper draws a conclusion that the Japanese home islands will not sink as long as earthquake still exists, volcano is still active and their lava cones are still there. In this paper, based on the independence of seismic cone tectonic activities, the general relationship between strong earthquakes, volcanoes and seismic cone tectonics in Japan and its adjacent areas is preliminarily clarified. Relying on the mature seismic data in Japan, Interested parties may be able to find out the prediction methods of intracrustal strong earthquakes and seismic cone type volcanism as soon as possible through the principle and working method of Seismo-Geothermics.

Ring-shaped seismicity structures in the region of Kamchatka: possible preparation for great earthquake

We have been studying some seismicity characteristics in the region of Kamchatka region. It was established that two pairs of ring-shaped seismicity structures at depths of 0-33 and 34-70 km have been formed here in 1973-2018 prior to large earthquakes (05.12.1997 г., Mw=7.8 and 20.12.2018, Mw=7.3). Ring-shaped structures are characterized by threshold magnitude values (Mt1 and Mt2 correspondingly) and also big axes length (L1 and L2). Ring-shaped structures were identified in the area to the east and northeast of Petropavlovsk-Kamchatskiy city, where no great earthquakes have occurred during more than 60 years (ring-shaped structure was identified also in the depth range of 71-110 km here). Earlier, correlation dependences of Mt1 and Mt2 parameters on Mw values of major earthquakes for the west of Pacific were creating. Using these dependencies, the magnitude of possible great earthquake in this area was estimated as Mw=8.6±0.2. Besides that, a conclusion on the source depth of such event was drawing on the base of the analysis of three ring-shaped structures. The reasons for ring-shaped structures formation in different depth ranges of the subduction zones are discussing.

Applying machine learning to characterize and extrapolate the relationship between seismic structure and surface heat flow

SUMMARY Geothermal heat flow beneath the Greenland and Antarctic ice sheets is an important boundary condition for ice sheet dynamics, but is rarely measured directly and therefore is inferred indirectly from proxies (e.g. seismic structure, magnetic Curie depth, surface topography). We seek to improve the understanding of the relationship between heat flow and one such proxy—seismic structure—and determine how well heat flow data can be predicted from the structure (the characterization problem). We also seek to quantify the extent to which this relationship can be extrapolated from one continent to another (the transportability problem). To address these problems, we use direct heat flow observations and new seismic structural information in the contiguous United States and Europe, and construct three Machine Learning models of the relationship with different levels of complexity (Linear Regression, Decision Tree and Random Forest). We compare these models in terms of their interpretability, the predicted heat flow accuracy within a continent and the accuracy of the extrapolation between Europe and the United States. The Random Forest and Decision Tree models are the most accurate within a continent, while the Linear Regression and Decision Tree models are the most accurate upon extrapolation between continents. The Decision Tree model uniquely illuminates the regional variations of the relationship between heat flow and seismic structure. From the Decision Tree model, uppermost mantle shear wave speed, crustal shear wave speed and Moho depth together explain more than half of the observed heat flow variations in both the United States [$r^2 \approx 0.6$ (coefficient of determination), $\mathrm{RMSE} \approx 8\, {\rm mW}\,{\rm m}^{-2}$ (Root Mean Squared Error)] and Europe ($r^2 \approx 0.5, \mathrm{RMSE} \approx 13\, {\rm mW}\,{\rm m}^{-2}$), such that uppermost mantle shear wave speed is the most important. Extrapolating the U.S.-trained models to Europe reasonably predicts the geographical distribution of heat flow [$\rho = 0.48$ (correlation coefficient)], but not the absolute amplitude of the variations ($r^2 = 0.17$), similarly from Europe to the United States ($\rho = 0.66, r^2 = 0.24$). The deterioration of accuracy upon extrapolation is caused by differences between the continents in how seismic structure is imaged, the heat flow data and intrinsic crustal radiogenic heat production. Our methods have the potential to improve the reliability and resolution of heat flow inferences across Antarctica and the validation and cross-validation procedures we present can be applied to heat flow proxies other than seismic structure, which may help resolve inconsistencies between existing subglacial heat flow values inferred using different proxies.

Stress drops of intermediate-depth intraslab earthquakes beneath Tohoku, northern Japan

We calculated stress drops for 2875 small intraslab earthquakes at intermediate depths beneath Tohoku, Japan. We applied an S-coda-wave spectral ratio method to almost 900,000 event pairs. Detailed velocity values for the oceanic crust (OC) were adopted from previous observational studies. The median stress drops in the OC are about half those in the oceanic mantle (OM). The median stress drop for earthquakes in the OC decreases from depths of 70 to 120 km and increases from 120 to 170 km. Our preferred interpretation is that the rigidity in the OC decreases and then increases with depth due to combined effects of the dehydration associated with the eclogite formation and the increasing temperature with depth. These depth variations are consistent with results of a similar study beneath Hokkaido. The median stress drops in the oceanic plate beneath Tohoku are generally smaller than those beneath Hokkaido. Previous studies imaging the seismic structure at shallow depths and b-value analyses of intraslab earthquakes indicate that the near-trench region of the oceanic plate off Tohoku is more hydrated than that off Hokkaido. Taken together, these results suggest that differences in the degree of hydration of the oceanic plate in the near-trench regions could produce the different behaviors of stress drops of intermediate-depth earthquakes observed in Tohoku and Hokkaido.Graphical

Contrastive Investigation of High rise building with distinctive infill wall by Pushover analysis

Pushover analysis is a method that uses simple nonlinear techniques to predict seismic structural deformations. Today, we use masonry infill in reinforced concrete (R/C) frames for architectural, aesthetic or economic reasons. In this project, we need to study the effect of backfill on the damage structure of the reinforced concrete frame. The main purpose of this study is to show that adding walls to the reinforced concrete frame can increase the strength and stiffness of seismic resistant structure loads and increase the feedback for strength and stiffness analysis.These instructions strictly comply with FEMA-356. In this project, we use three types of bricks: red brick, fly ash brick, deep brick and siporex brick. Taking the output of non-linear analysis, we compare layer V/S i) Base Shear, ii) Storey Displacement, iii) Floor Shift Base Shear V/S Attack and Observe Spectrum Acceleration V/S spectral function . We also use ETABS 2017 software to study the effects of bare shear walls.. Key Words: Pushover Examination, Brick infill, FEMA-356, Displacement, Float, Shear Divider, ETAB-2017

Response Modification Factor of High-Strength Steel Frames with D-Eccentric Brace Using the IDA Method

The design innovation of high-strength steel frames paired with D-eccentric bracing exhibits remarkable resistance to plastic deformation during seismic events. This method strategically combines regular steel connections (with yield strengths below 345 MPa) and high-strength steel beams and columns (such as Q460 or Q690, with yield strengths over 460 MPa), effectively reducing cross-sectional sizes while preserving the elasticity of non-energy-dissipating members. This configuration results in substantial ductility and superior energy dissipation capabilities. The response modification factor (R) is vital for achieving both effective and economical seismic resilience, particularly in the development of efficient and cost-effective seismic designs. However, the 2016 edition of the Code for Seismic Design of Buildings (GB50011-2010) fails to incorporate the concept of R, opting instead to apply a uniform value to all structural systems. This oversight is fundamentally flawed, necessitating a comprehensive investigation into the R value specifically for the high-strength steel frame with a D-eccentric brace. This research primarily aims to improve structural performance design, provide guidance for future projects, and encourage the adoption of this advanced seismic performance structure in earthquake-prone areas. To achieve these objectives, a performance-based seismic design approach is employed. This method involves designing structures with varying numbers of stories (4, 8, and 12), different link lengths (900, 1000, and 1100 mm), and various steel strengths (Q460 and Q690). This study uses the Incremental Dynamic Analysis (IDA) method to determine the R values for each prototype. The derived performance coefficients act as crucial references for the development of future innovative structural designs. This research greatly enhances seismic design practices and facilitates the wider adoption of high-strength steel frames with D-eccentric braces due to their outstanding seismic performance.

The KISS Network in 2015–2016: Catalogs and Comparison of the Processing Results with Operational Estimates from the Permanent Network

As part of the international collaboration of several research groups from Russia, France, and Germany, 77 temporary seismic stations were installed in the summer of 2015 for one-year period to conduct a detailed study of the deep structure of the Earth’s crust and upper mantle in the region of the Klyuchevskoi Volcano Group (KGV) in the Kamchatka Peninsula. One of the results of the KISS experiment (Klyuchevskoi Investigation – Seismic Structure of an extraordinary volcanic system) was the final catalog of the joint data from the temporary stations and the permanent network of the Kamchatka Branch of the Geophysical Survey of the Russian Academy of Sciences (KB GS RAS). The catalog comprises 2136 events, including 560 for which the permanent network catalog lacked sufficient data for correct processing. The catalog in .xlsx format and the station bulletin in .isf format are presented in the supplementary material to the paper. A comparative analysis was conducted on the joint solutions of two catalogs: one obtained solely from the data of the KB GS RAS permanent network stations and another from a denser seismic network integrated with KISS stations.

Ancient Craton‐Wide Mid‐Lithosphere Discontinuity Controlled by Pargasite Channels

AbstractThe mechanisms governing a commonly observed seismic velocity drop in the cratonic lithosphere, referred to as the mid‐lithospheric discontinuity (MLD), have been widely debated. To identify the composition and seismic structure of MLDs, we have analyzed Sp receiver functions (SRF) and mantle xenocrysts for six regions across Australia. We utilize locations where seismic stations and kimberlite‐hosted mantle xenocrysts are both available, allowing for comparison between seismological and petrological constraints. Our results show negative SRF phases indicative of the MLD coincide with clinopyroxene‐depleted zones at 60–140 km depth. Clinopyroxenes with different chemical compositions across the MLD define a litho‐chemical discontinuity. Modeling and experimental data show that MLDs may be explained by modified lherzolite with 10%–20% modal pargasite. Pargasite MLDs may form when rising H2O‐bearing melts cross the amphibole dehydration curve and react with clinopyroxene in lherzolite. Because the amphibole dehydration curve is isobaric at 80–120 km, pargasite will be precipitated as horizontal channels.

Seismic Imaging of Shallow Depths using Ambient Noise Tomography in the Hyderabad Region, Eastern Dharwar Craton, India

ABSTRACT Hyderabad and its surrounding region are situated within Eastern Dharwar Craton, primarily composed of Archean granite. We use ambient noise tomography to obtain the 3-D shallow subsurface shear seismic velocity structure of 150 x 200 km2 area around Hyderabad. We used continuous waveforms that were recorded by a digital seismic network with 10 three-component broadband seismographs from November 2020 to December 2021. Using the vertical component of waveforms, the Empirical Green’s functions (EGFs) have been estimated, and the group velocity dispersion curves in the time period range of 1.0–10 s have been extracted from these. We inverted these dispersion curves to estimate 3-D variation of shear wave velocity by using direct surface wave tomographic method. The results suggest that the subsurface velocity structure is heterogeneous. The shear wave velocity varies from 3.4 to 3.7 km/s in the shallow subsurface. The limited seismicity in this area is mostly confined to the boundary between high and low-velocity changes.

Multi-Objective Optimization for Seismic Structural Investigation and Maintenance Strategies of Bridges in Service

With the continuous updating of seismic design codes for Bridges, the new regulations have higher requirements for the seismic structure and mechanical properties of old Bridges. In order to make the decision of seismic structure strengthening more scientifically, with the goal of optimal overall performance, minimum economic cost and minimum environmental impact, the optimal model of seismic strengthening sustainable strategy is established. Firstly, according to the results of the seismic measures of the existing Bridges, the corresponding reinforcement measures are proposed for the structural problems that do not meet the seismic measures. Then the utility evaluation of the three objectives of the existing bridge was carried out. At last, NSGA-Ⅱ algorithm was used to build a multi-objective maintenance decision optimization model of the bridge network. Combined with the weight of the decision-maker's preference, TOPSIS algorithm was used to obtain the optimal solution of the target, and decision-making suggestions were provided for the relevant departments. The results show that there are many structural problems in the new evaluation of the old bridge, and the maintenance of the structural problems are more feasible and effective than the improvement of the traditional mechanical properties. Combined with the comprehensive evaluation utility and reinforcement objective function, the maintenance decision can be better under different objectives. The reinforcement maintenance decision model has the ability to improve the comprehensive performance and sustainability of the bridge to the maximum extent within the limited budget.

Considerations for the static and seismic design of pumped storage reservoirs in soft soils and seismic environments

AbstractFor recent hydroelectric pumped storage projects, the construction of embankment dam structures on alluvial deposits consisting of exceptionally soft soil and in areas with high seismicity was required. This contribution shares experiences and challenges from the static and seismic design of embankment dam structures and ground treatment options. Finite element‐based static and seismic analyses using the Hardening Soil Small Strain model (HSS) showed that preconsolidation has clear benefits compared to stone columns for ground improvement purposes. Regardless of general advantages of surface sealing systems for frequent and rapid impoundment level changes, it was found that clay core design options perform very well under seismic loading conditions and even better than surface‐sealed structures. Seismic ground and structure response analyses involving soft soil considering shear strain‐dependent stiffness degradation revealed that deamplification is expected in contrast to the rule‐of‐thumb amplification approaches enshrined in many seismic design guidelines and codes. It was found that this effect increases with seismic magnitudes and that saturation of peak ground acceleration (PGA) takes place. This results in relatively moderate structure excitations even under significant dynamic excitation.

1300-m-High gas plume from pockmarks in the north Nansha waters, South China sea

The northeastern region of Nansha Waters, located in the southern margin of the South China Sea, is abundant in gas resources. However, it remains unclear if the central and northern areas of Nansha Waters possess gas resources and if there are any associated leaks. The Jiuzhang Basin is located in the northern Nansha Waters. Pre-stack depth migration, analysis of seismic attributes and multibeam data processing were conducted to image the seabed topography and backscatter intensity, as well as the seismic structure of the water and sedimentary layers in the Jiuzhang Basin. The result reveals a presence of large plumes, approximately 1500 m in width and up to 1300 m in height, from a pockmark. The plume is characterized by a strong seismic amplitude featuring chaotic reflections, a high instantaneous frequency (100–150 Hz), and a strong instantaneous amplitude. These seismic features commonly appear in hydrocarbon-bearing seawater, indicating that the plume is composed of gas. The relationship between the volume fraction of bubbles in the gas plumes and the amplitude of multi-channel seismic waves suggests that the volume fraction of bubbles within the gas plume can reach up to 5.0 × 10−3. Meanwhile, a new pockmark with a length of 248 m, a width of 90 m, and a depth of 20.4 m, and an abrupt 22 m drop in water depth of an existing pockmark are observed in the gas plume area. These observations, combined with the gas plume revealed by the multi-channel seismic collected in 2013 and multibeam collected in 2020, suggest a very strong and recently active gas leaks in the pockmark area of the northern Jiuzhang Basin. Furthermore, in the pockmark area, there are gas-bearing formations with weak reflections, and mud diapirs charactered by low seismic velocity, low amplitude chaotic reflections, and pulled up reflections on both sides, and faults extend to the seabed. These observations suggest that gas plume originate from deep strata of the northern Jiuzhang Basin. Large gas plumes and mud diapirs are often indicative of the existence of deep gas reservoirs. Therefore, the presence of such active gas plumes and mud diapirs in the northern region of the Jiuzhang Basin signifies the presence of abundant gas resources.

DG-based joint transmission-reflection traveltime tomography and its application of borehole seismic

Abstract The limitations of the coverage range and density of transmission wave often result in less-than-ideal results in traveltime tomography. In contrast, joint transmission-reflection traveltime tomography can not only recover deep structures that transmission tomography cannot detect but also optimize its inversion results. In this article, we perform joint tomography on borehole seismic (VSP, RVSP and crosswell seismic) data to obtain near-wellbore structures. In the forward part, we solve the factored equation by the discontinuous Galerkin (DG) method to calculate the transmission/reflection traveltime. Due to the large wavefront curvature near the source point, the traveltime errors generated by the numerical simulation will propagate from the source to all the calculation domains. According to the factorization principle, the equation solution is decomposed into two parts to solve the point-source singularity. To further improve the accuracy of solving traveltime, we use the DG method to solve the factored eikonal equation with additive factors (the factored DG method), obtaining second-order accuracy solution. The adjoint-state method is employed in the inversion section to calculate the gradient of the misfit function. And we use the traveltime difference observed inside the model to define the misfit function, which is more suitable for borehole seismic and avoids the influence of surface normal vectors on gradients. Numerical tests applied on models indicate that the joint tomography method has the potential to accurately inverse the seismic structure information near the well and recover the deep underground structure.

Seismic structure of the 2015 Mw7.8 Gorkha earthquake revealed by ambient seismic noise and teleseismic surface wave tomography

The destructive 2015 Mw7.8 Gorkha earthquake occurred in the Main Himalayan Thrust due to the collision of the Indian and Asian plates, which provides a unique opportunity to understand the deep dynamic processes and seismogenic mechanisms of strong earthquakes. We construct a regional-scale shear-wave velocity model of the crust and uppermost mantle using ambient seismic noise and teleseismic surface wave at periods of 5–100 s around the Gorkha earthquake region. The new shear-wave velocity model exhibits prominently lateral heterogeneities in the Gorkha earthquake areas. We observe a high-velocity (high-V) zone around the Gorkha main shock in the Main Himalayan Thrust, indicating the existence of a high-strength asperity that sustains the stress accumulating. The aftershocks are primarily located in the low-velocity (low-V) anomalies and enclosed by two high-V anomalies, which appear to act as structural barriers that influence the spread of the aftershocks. Prominent low-Vanomalies from the lower crust to the mantle lithosphere are observed along the north–south trending rifts, suggesting the hot materials upwelling due to the tearing of the northward subducting Indian lithosphere. These observations may indicate that seismic velocity heterogeneity could play an essential role in earthquake initiation and the rupture process.

Thermodynamics of mantle minerals – III: the role of iron

SUMMARY We expand the scope of HeFESTo by encompassing the rich physics of iron in the mantle, including the existence of multiple valence and spin states. In our previous papers, we considered iron only in its most common state in the mantle: the high-spin divalent (ferrous) cation. We now add ferric iron end-members to six phases, as well as the three phases of native iron. We also add low-spin states of ferrous and ferric iron and capture the behaviour of the high-spin to low-spin transition. Consideration of the multi-state nature of iron, unique among the major elements, leads to developments of our theory, including generalization of the chemical potential to account for the possibility of multiple distinguishable states of iron co-existing on a single crystallographic site, the effect of the high-spin to low-spin transition on seismic wave velocities in multiphase systems, and computation of oxygen fugacity. Consideration of ferric iron also motivates the addition of the chromia component to several phases, so that we now consider the set of components: Ca, Na, Fe, Mg, Al, Si, O and Cr (CNFMASO+Cr). We present the results of a new global inversion of mineral properties and compare our results to experimental observations over the entire pressure–temperature range of the mantle and over a wide range of oxygen fugacity. Applications of our method illustrate how it might be used to better understand the seismic structure, dynamics and oxygen fugacity of the mantle.

The Deep Seismic Structure of the Pacific Tectonic Belt along the 8-DV Reference Profile

The Deep Seismic Structure of the Pacific Tectonic Belt along the 8-DV Reference Profile

Post-collisional lithospheric delamination in eastern Iran, revealed by non-linear teleseismic tomography and residual topography

The Eastern Iranian Mountain Ranges (EIR) emerged as a consequence of the Late Cretaceous collision between the Afghan and Lut blocks. However, the response of the uppermost mantle to this collision remains enigmatic. Additionally, although petrological evidence suggests that post-collisional delamination is possible, it has not been conclusively identified in prior regional seismic imagery. This observation leads us to further explore this possibility using a dense seismic network. To gain insight into the geodynamic implications for eastern Iran and address knowledge gaps, we extensively investigated the seismic structure of the uppermost mantle beneath the EIR using a dense seismic network of 34 temporary stations, complemented by data from nine additional local permanent stations. By meticulously analyzing 6589 relative arrival time residuals from teleseismic records with favorable signal-to-noise ratios, we applied a non-linear tomography method to map P-wave velocity perturbations in a relative sense. Our tomographic images unveiled distinct instances of rapid high-velocity anomalies beneath low-velocity regions in the shallow mantle, suggesting the potential occurrence of lithospheric dripping, followed by subsequent asthenospheric upwelling. This observation offers a plausible explanation for the observed post-collisional magmatism over the Lut Block. Furthermore, to maintain the approximately 1.5-km positive residual topography across the EIR, beyond the influence of crustal properties, additional support from the hot and buoyant asthenosphere becomes crucial, particularly in the absence of a substantial lithospheric mantle.

Detailed seismic structure beneath the earthquake zone of Yogyakarta 2006 (Mw ∼6.4), Indonesia, from local earthquake tomography

The earthquake, which occurred in Yogyakarta, Indonesia, on May 26, 2006, at 22:53:58 UTC with Mw ∼6.4, was one of the most destructive earthquakes in Indonesia. The earthquake caused thousands of fatalities, tens of thousands of injuries, and hundreds of thousands of house damages in the Yogyakarta area and its surroundings at a loss of billions of dollars. Previous studies from seismic tomography and satellite radar imaging hypothesized that the earthquake was caused by activating a so far unknown fault east of the Opak Fault. Although, in the beginning, the Opak fault was suspected to be the source of the Yogyakarta earthquake in 2006. This assumption was made because the damage was maximum in the Bantul area west of the Opak Fault. This study demonstrates that our seismic tomography achieved a higher resolution than the previous study and could resolve a failed complex fault system. We utilized more aftershocks (2170 events) and smaller grid sizes for seismic tomography inversion. Four focal mechanisms from aftershocks for Mw ≥ 4.5 were also conducted to support structure interpretation in the study area. Our results successfully delineate the Opak Fault and the second fault, namely the Ngalang Fault, parallel to the eastern part of the fault at a depth of 9 km. Two faults could be indicated by the velocity contrast of Vp, Vp/Vs ratio, and Vs from a horizontal section tomogram. Our focal mechanisms also support seismic tomography, revealing two fault planes in our study area. The results show that the two faults are connected by the Oyo Fault, which is ruptured in the opposite direction compared to the two faults.

Depositional pattern and source-to-sink process of submarine fans in Lingshui and Sanya formations, Baodao Sag, Qiongdongnan Basin, China

The deepwater area is one of the frontiers of oil and gas exploration, with the understanding of distribution and genesis of deepwater reservoirs being crucial for reservoir formation research. Despite Baodao Sag in the Qiongdongnan Basin being proven to be rich in hydrocarbon generation, significant oil and gas fields have yet to be discovered in the ultra-deepwater areas, and the distribution patterns of reservoirs in deep water remain unclear. Taking the southern slope area of the Baodao Sag as an example, the distribution characteristics of submarine fans are studied by employing seismic prediction methods, including seismic reflection structure analysis, seismic facies geometry, seismic attribute analysis, and the source-to-sink theory. The results show distinct characteristics: the fan delta exhibits parallel oblique progradational reflection, the slumping submarine fan displays lenticular reflection, and the submarine fan of the Sanya Formation demonstrates subparallel sheet reflection. The provenance of sediment is traced back to the denudation area of the Songnan low uplift and the Southern Uplift area in the southwest. The sediments were primarily transported through two main incised channel systems in the north and south, ultimately flowing into the southern slope area of the Baodao Sag. The application of seismic sedimentological prediction methods and source-sink theory has laid a solid geological foundation for oil and gas exploration and the analysis of reservoir forming conditions in the deepwater area of the Baodao Sag.

Crustal structure of Borneo, Makassar Strait and Sulawesi from ambient noise tomography

SUMMARY Borneo and Sulawesi are two large islands separated by the Makassar Strait that lie within the complex tectonic setting of central Indonesia. The seismic structure beneath this region is poorly understood due to the limited data availability. In this study, we present Rayleigh wave tomography results that illuminate the underlying crustal structure. Group velocity is retrieved from dispersion analysis of Rayleigh waves extracted from the ambient noise field by cross-correlating long-term recordings from 108 seismic stations over a period of 8 months. We then produce a 3-D shear wave velocity model via a two-stage process in which group velocity maps are computed across a range of periods and then sampled over a dense grid of points to produce pseudo-dispersion curves; these dispersion curves are then separately inverted for 1-D shear wave velocity (Vs), with the resultant models combined and interpolated to form a 3-D model. In this model, we observed up to ± 1.2 km s−1 lateral Vs heterogeneities as a function of depth. Our models illuminate a strong low shear wave velocity (Vs) anomaly at shallow depth (≤ 14 km) and a strong high Vs anomaly at depths of 20–30 km beneath the North Makassar Strait. We inferred the sediment basement and Moho depth from our 3-D Vs model based on iso-velocity constrained by the positive vertical gradient of the Vs models. The broad and deep sedimentary basement at ∼14 ± 2 km depth beneath the North Makassar Strait is floored by a shallow Moho at ∼22 ± 2 km depth, which is the thinnest crust in the study area. To the east of this region, our model reveals a Moho depth of ∼45 ± 2 km beneath Central Sulawesi, the thickest crust in our study area, which suggests crustal thickening since the late Oligocene. Moreover, the presence of high near-surface Vs anomalies with only slight changes of velocity with increasing depth in southwest Borneo close to Schwaner Mountain confirm the existence of a crustal root beneath this region.